The invention relates to improvements in devices for guiding the thread, particularly the needle thread, in so-called central bobbin (CB) or oscillating hook type sewing machines. In such machines, the shuttle hook oscillates in a substantially vertical plane.
An advantage of sewing machines which operate with CB hooks is that the needle thread loop is not twisted while it is being trained around the hook and around the bobbin case, as well as that the thread can readily pass between the hook and the shuttle driver finger, merely as a result of a reversal in the direction of movement of the hook and driver finger, without the need for any specially designed auxiliary equipment. Consequently, such hooks are normally less sensitive to changes in the tension of the thread and they can assist in the making of eye-pleasing uniform stitches.
Friction between the limbs of the needle thread loop develops in a CB hook type sewing machine the same as in machines employing other types of shuttle hooks. The magnitude of friction between the (work and takeup) limbs of the needle thread loop depends upon a variety of parameters including the diameters and types of threads, the selected type of stitching, the length of the loops, the width of the stitches, the speed of the sewing machine and others. Under certain circumstances, such friction can adversely affect the quality of the stitches.
The loop which is formed by the needle thread in a CB hook type sewing machine must completely surround the shuttle hook, the bobbin case and the supply of bobbin thread in the case. The loop is cast off shortly or immediately before the hook changes the direction of its movement, and the thus released loop is lifted by the takeup lever to be drawn into the work. As a rule, each upward movement of the takeup lever entails an upward movement of the front or work limb of the needle thread loop, i.e., of that limb which extends to the eye of the needle. The rear or takeup limb of the needle thread loop, which extends from the needle to the work and is located behind the hook, remains more or less passive. That portion of the bobbin thread which extends from the bobbin case to the underside of the work passes between the two limbs of the needle thread loop and normally does not interfere with a predictable reduction of the size of the needle thread loop. However, when the machine is set to make long and/or wide stitches, the needle penetrates behind that portion of the bobbin thread which extends from the bobbin case to the work during transition from a right downward stroke to a left downward stroke. This entails a looping of needle thread around the bobbin thread following the castoff from the hook with the result that additional friction develops in the region of the work limb of the needle thread loop. This, in turn, results in a more rapid upward movement of the takeup limb of the needle thread loop. The just described mode of operation is acceptable due to the fact that it does not appreciably affect the quality and/or appearance of stitches when the sewing machine is operated at a medium speed or at an elevated speed because the making of stitches takes place at frequent intervals and the inertia of the takeup limb of the needle thread loop is rather pronounced so that the takeup limb cannot react in response to development of additional friction with the work limb. However, frictional engagement between the limbs of the needle thread loop lasts longer, and the movements of such limbs relative to each other are slower, when the sewing machine is operated at less than average speed. At such time, the static component of friction is more pronounced and affects the thread much more than the dynamic component of friction. The upward movement of the takeup limb of the needle thread loop is too rapid so that the entire needle thread is not drawn all the way into the work and the work is provided with so-called slack or loose stitches. The making of slack stitches takes place at random so that they greatly affect the appearance of the product.
German Utility Model No. 70 16 286 discloses an oscillating hook type sewing machine wherein the needle thread loop is guided by a horn-shaped arm and by a holding finger which cooperates with the arm as soon as the loop is cast off by the circulating hook. This is intended to prevent the development of friction between the limbs of the needle thread loop by preventing the limbs from coming into contact with each other as well as to prevent an irregular or unpredictable reduction of the size of the needle thread loop. Such undertakings (see also U.S. Pat. No. 4,095,539) are satisfactory in sewing machines which employ circulating (rotary) looptakers. However, the teachings of these references cannot be incorporated in CB hook type sewing machines wherein the hook performs an oscillatory (back and forth) rather than a rotary movement.
German Pat. No. 423,990 proposes to provide the bobbin case in a sewing machine utilizing a circulating shuttle hook with a projecting nose serving to brake that (work) limb of the needle thread loop which extends toward the work until the other limb (which extends toward the source of the needle thread) has passed between the bobbin case and the retaining finger. A similar proposal is disclosed in German Offenlegungsschrift No. 33 42 770 which discloses a spring to be mounted on the shuttle driver in order to brake the takeup limb of the needle thread loop while the needle is caused to move upwardly. These proposals can be relied upon to prevent, at least to a considerable degree, the upward movement of the takeup limb of the needle thread loop. However, they also exhibit a serious drawback, namely that they effect an abrupt release of the needle thread loop which often leads to irregularities in the stitching pattern.